The invention relates in general to a semiconductor device, and in particular relates to a semiconductor device which is suitable for application as a device provided with functions to monitor thermal resistance.
In power semiconductor devices, a semiconductor chip is mounted on a copper base with high thermal conductivity so as to efficiently dissipate heat from the semiconductor chip, with a ceramic substrate for insulation interposed, and solder is used for bonding of members between the semiconductor chip and the copper base. These members have large differences in thermal expansion coefficient, and stresses and strains occur between these members due to temperature cycles, so that the solder undergoes fatigue, and solder cracks occur. When a solder crack occurs, heat dissipation from the semiconductor chip is degraded, and the temperature of the semiconductor chip rises rapidly, leading to the danger of thermal failure of the semiconductor chip.
In order to avoid such danger in advance, life tests of semiconductor elements are performed to determine life acceleration factors, and by predicting the lifetime of a semiconductor element, the semiconductor device can be operated within the range of the predicted lifetime. Alternatively, a method can be used in which the temperature of the semiconductor chip is monitored, and upon detecting an anomalous rise in temperature, operation of the semiconductor device is forcibly halted.
FIG. 7 is a block diagram showing in summary the configuration of a conventional semiconductor device. In FIG. 7, a semiconductor chip 11 is provided on which is formed a switching element 12 which performs switching operation and a diode 13 used in detection of the temperature of the semiconductor chip 11. As the switching element 12, an IGBT (Insulated Gate Bipolar Transistor) can be used.
If an IGBT is used as the switching element 12, the emitter, with current divided from the IGBT collector current, is grounded via the resistance 18, and the gate of the IGBT is connected to the output terminal of a logical AND circuit 24 via a driving circuit 25, while the diode 13 is connected to the input terminal of a temperature calculation portion 17. The output terminal of the temperature calculation portion 17 is connected to the inverting input terminal of a comparator 23, a reference voltage source 22 is connected to the non-inverting input terminal of the comparator 23, the output terminal of the comparator 23 is connected to the inverting input terminal of the logical AND circuit 24, the non-inverting input terminal of the logical AND circuit 24 is connected to a signal input terminal, and the output terminal of the logical AND circuit 24 is connected to the input terminal of the driving circuit 25.
When a control signal which controls switching operation of the switching element 12 is applied to the gate of the switching element 12 via the logical AND circuit 24 and driving circuit 25, the switching element 12 is turned on, and current flows in the resistance 18. Then, as the temperature of the semiconductor chip 11 rises accompanying switching operation of the switching element 12, the current flowing in the diode 13 changes according to the temperature characteristic of the diode 13, and the signal from this diode 13 is input to the temperature calculation portion 17.
The temperature calculation portion 17 calculates the temperature of the semiconductor chip 11 based on the signal from the diode 13, and inputs a voltage corresponding to this temperature to the comparator 23. The comparator 23 compares the voltage corresponding to the temperature of the semiconductor chip 11 with a reference voltage, and if the temperature of the semiconductor chip 11 is equal to or above a prescribed value, sets the inverting input terminal of the logical AND circuit 24 to low level, to impede the input of the control signal to the gate of the switching element 12, and halt operation of the switching element 12.
For example, in Japanese Patent Laid-open No. H7-14948, a method is disclosed in which, by installing a thermocouple on a semiconductor element and components, the temperature during device use is constantly monitored, and degradation of various bonded portions and the state of cooling of the device can be ascertained.
Further, in for example Japanese Patent Laid-open No. H9-148523 (corresponding to U.S. Pat. No. 5,721,455), a method is disclosed in which an element is provided which detects increases in thermal resistance in a heat dissipation path which releases generated heat in a state of use of a semiconductor element, and in which output of the detection result to equipment exterior to the semiconductor device enables early detection of fragility due to the occurrence of cracks in solder which fix the insulating substrate for element mounting and the heat-dissipating substrate in a state of use.
Still further, in International Patent Application Publication No. WO2005/038919, a method is disclosed in which a metal wire is used to detect the characteristics of a bonded portion which bonds metals, and by using a threshold value determined from the relation between lifetime and an increase in resistance due to degradation of the bonded portion, degradation of the bonded portion is predicted.
However, in methods to predict the lifetime of a semiconductor element, there is the problem that, if the model case used in lifetime prediction differs from the actual operation pattern during operation, lifetime prediction may not be possible, or it may not be possible to avoid the danger that thermal failure of the semiconductor chip.
Further, in methods to monitor the temperature of the semiconductor chip, there is the problem that, when the semiconductor device operates at or below the rated current, even when a solder crack occurs, an anomalous increase in temperature of the semiconductor chip cannot be detected, and thermal failure of the semiconductor chip cannot be prevented in advance.
Hence, it would be desirable to provide a semiconductor device that enables detection of degradation of heat dissipation from a semiconductor chip, even in cases in which the operating state of the semiconductor device cannot be predicted.